A liquid chromatograph mass spectrometer (LC/MS) is formed of a liquid chromatograph unit (LC unit) for separating a liquid sample into its respective ingredients that are then eluted, an ionization chamber (interface unit) for ionizing sample ingredients that have eluted from the LC unit, and a mass spectrometry unit (MS unit) for detecting the ions that have been introduced from the ionization chamber. In such an ionization chamber, various ionization techniques are used in order to ionize sample ingredients, and atmospheric pressure ionization methods such as atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) are widely used.
In typical APCI, the end of a nozzle that is connected to the terminal of a column in the LC unit is arranged so as to be directed toward the inside of the ionization chamber, and at the same time, a needle electrode is placed in front of the end of the nozzle. Thus, a droplet of the sample that has been atomized through the application of heat in the nozzle is chemically responded to carrier gas ions (buffer ions) that have been generated through corona discharge from the needle electrode and is ionized. In ESI, the end of a nozzle that is connected to the terminal of a column in the LC unit is arranged so as to be directed toward the inside of the ionization chamber, and at the same time, a high voltage of approximately several kV is applied to the end of the nozzle so as to generate a strong non-uniform electric field. As a result, the sample ingredients are charge separated by the electric field and torn off due to Coulomb attraction, and thus are atomized. Consequently, the solvent in the droplet of the sample is evaporated upon contact with the ambient air, and thus, gas ions are generated.
In the above-described APCI and ESI, the sample ingredients are ionized in a state that is close to the atmospheric pressure, and therefore, a difference in the pressure is secured between the ionization chamber in a high pressure state (that is to say, a state that is close to the atmospheric pressure) and the MS unit in a state of very low pressure (that is to say, a state that is highly vacuumed), and thus, such a configuration is adopted that middle chambers are provided between the ionization chamber and the MS unit so that the degree of vacuuming is increased step by step.
FIG. 4 is a schematic diagram illustrating the structure of an example of a liquid chromatograph mass spectrometer using an ESI method. Here, the forward direction that is parallel to the ground is the X direction, the direction that is parallel to the ground and perpendicular to the X direction is the Y direction, and the direction that is perpendicular to the X direction and the Y direction is the Z direction.
A liquid chromatograph mass spectrometer 201 is provided with an LC unit 2, a probe (ion source) 15, an ionization chamber 11 having a chamber (housing) 210, a first middle chamber 12 that is adjacent to the ionization chamber 11, a second middle chamber 13 that is adjacent to the first middle chamber 12, a mass spectrometry chamber (MS unit) 14 that is adjacent to the second middle chamber 13, and a computer 240 that controls the entirety of the liquid chromatograph mass spectrometer 201.
Each sample to be measured (sample ingredient) that is gained by separating a sample into ingredients in the LC unit 2 is supplied through a sample to be measured flow path 155, and a nebulizing gas (nitrogen gas) is supplied through a nebulizing gas flow path 156. As a result, the liquid sample and the nebulizing gas are led to a probe 15 so as to be sprayed.
FIG. 5A is a side diagram showing a probe, and FIG. 5B is a cross-sectional diagram showing an enlargement of A in FIG. 5A. The probe 15 has a double tube structure where a sample to be measured that has been supplied through the sample to be measure flow path 155 is sprayed from the inside of a circular tube 151. Meanwhile, a nitrogen gas that has been supplied through the nebulizing gas flow path 156 is sprayed between the circular tube 151 and a nozzle 152 in a circular tube form. As a result, the sprayed sample to be measured is sprayed in an atomized state resulting from the collision effects with the nebulizing gas that is sprayed from the side around the circular tube 151.
Though not shown, wires are connected so that a high voltage of several kV can be applied to the end of the nozzle 152 from the voltage supply, and such a configuration makes ionization possible. The housing of the ionization chamber 11 is a chamber 210 in a rectangular parallelepiped form of 13 cm×13 cm×12 cm, which has an upper wall, a partition (rear wall), a front wall, a right side wall, a left side wall and a lower wall. Thus, the space inside the ionization chamber 11 is surrounded by the upper wall, the partition (rear wall), the front wall, the right side wall, the left side wall and the lower wall.
A circular opening that runs in the forward and backward direction (X direction) is created in the front wall so that a probe 15 can be attached in the opening.
The partition is provided between the ionization chamber 11 and the first middle chamber 12, and a solvent releasing tube (ion introducing tube) 19 in circular tube form (having an outer diameter of 1.6 mm and an inner diameter of 0.5 mm) is formed in the partition, and a dry gas flow path 50 is formed so as to cover the periphery of the solvent releasing tube 19. As a result, the inside of the ionization chamber 11 and the inside of the first middle chamber 12 communicate through the solvent releasing tube 19. In addition, the solvent releasing tube 19 functions to promote the release of the solvent and ionization through the application of heat and the collision effects when the ions and a droplet of a microscopic sample that have been sprayed by the probe 15 pass through the inside of the solvent releasing tube 19.
The nozzle of the probe 15 is directed toward the front (X direction) so as to face the entrance of the solvent releasing tube 19 with a predetermined distance (2 cm, for example) in between.
A drain 30 is created in the lower wall so that unnecessary samples can be discharged to the outside through the drain 30.
A first ion lens 21 is provided inside the first middle chamber 12, and an exhaust outlet 31 for vacuum exhaust using an oil-sealed rotary pump (RP) is provided in the lower wall of the first middle chamber 12. In addition, a skinner 22 having an orifice is formed in the partition between the first middle chamber 12 and the second middle chamber 13 so that the inside of the first middle chamber 12 and the inside of the second middle chamber 13 communicate through this orifice.
An octupole 23 and a focus lens 24 are provided inside the second middle chamber 13, and an exhaust outlet 32 for vacuum exhaust using a turbo molecular pump (TMP) is provided in the lower wall of the second middle chamber 13. In addition, an entrance lens 25 having an orifice is provided in the partition between the second middle chamber 13 and the mass spectrometry chamber 14 so that the inside of the second middle chamber 13 and the inside of the mass spectrometry chamber 14 communicate through this orifice.
A first quadrupole 16, a second quadrupole 17 and a detector 18 are provided inside the mass spectrometry chamber 14, and an exhaust outlet 33 for vacuum exhaust using a turbo molecular pump (TMP) is provided in the lower wall of the mass spectrometry chamber 14.
Here, the ion lens 21, the octupole 23, the focus lens 24 and the entrance lens 25 have convergence effects with which the ions that pass at respective ion velocities under the respective vacuum states are efficiently sent to the next stage.
In such a liquid chromatograph mass spectrometer, the ions generated in the ionization chamber 11 sequentially pass through the solvent releasing tube 19, the first ion lens 21 within the first middle chamber 12, the skinner 22, the octupole 23 and the focus lens 24 within the second middle chamber 13, and the entrance lens 25 so as to be sent to the mass spectrometry chamber 14, where the ions are separated depending on the size of the mass-to-charge ratio (m/z) by the quadrupoles 16 and 17 and reach the detector 18.
The computer 240 acquires intensity signals that correspond to the number of generated ions for each mass-to-charge ratio (m/z) so as to create a mass spectrum. As a result, the molecular weight of the sample to be measured is calculated from the mass-to-charge ratio (m/z) where a peak of the molecular ions appears in the mass spectrum. In addition, as for the fragment ions that are generated when the molecular ions are split, the manner of splitting is estimated from the mass-to-charge ratio (m/z) where a peak of each fragment ion type appears, and the molecular structure of the sample to be measured is analyzed.
Incidentally, in the above-described liquid chromatograph mass spectrometer 201, the value gained by measuring the mass spectrum changes as time elapses, and therefore, the mass-to-charge ratio (m/z) for each peak that is gained by measuring a sample is determined by using a peak that is gained by measuring a standard sample (calibration sample) of which the value is already known for a predetermined period of time (see Patent Literature 1). Typically, the difference (calibration value) between the value of the mass-to-charge ratio (m/z) for each peak that is gained by measuring a standard sample and the theoretical value of the mass-to-charge ratio (m/z) for the corresponding peak is calculated for each peak so that a calibration value can be found for any mass-to-charge ratio (m/z) by interpolating or extrapolating these calibration values. Thus, the calibration value is added to the value of the mass-to-charge ratio (m/z) for a peak that is gained by measuring a sample to be measured so that a precise mass-to-charge ratio (m/z) can be calculated for each peak.
In the case where mass spectrometry is carried out by mixing a sample to be measured and a standard sample (internal standard method), however, the efficiency in the ionization in the electrospray ion source (probe 15) is different between the respective samples, and in addition, only one sample is often ionized when mixed.
Therefore, a method for carrying out mass spectrometry on a sample to be measured and a standard sample “almost at the same time” or “separately” has been disclosed. Examples are a method for introducing a standard sample into the sample to be measured flow path 155 separately timewise (see Patent Literature 2), a method for mechanically switching the directions in which ions are introduced using a device for a number of types of ions (see Patent Literature 3), and a method for introducing a standard sample into a vacuum through a flow path that is different from the sample to be measured flow path 155.